1. Field of the Invention
The present invention relates generally to chemical mechanical planarization (CMP) systems and techniques for improving the performance and effectiveness of CMP operations. Specifically, the present invention relates to enhancing the performance of metal CMP systems.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform CMP operations, including planarization, buffing and wafer cleaning. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In some other applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization. Copper (Cu) CMP became a Cu Dual Damascene technology enabling operation as none of the other technologies are capable of shaping the copper plugs and wires.
CMP systems typically implement rotary, belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Normally, the removal of excess dielectric and metallization layers is achieved through in situ chemical modification of the processed wafer surface thus making the wafer surface more pliable for material removal. Slurry is used to facilitate and enhance the chemical modification of the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., pad, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The slurry distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface. Subsequently, the chemically modified excess metallization and dielectric layers are removed from the surface of the wafer. As the metallization and dielectric layers each have different chemical characteristics, the chemical mechanical planarization operation of the metallization layers defer from chemical mechanical planarization operation of dielectric layers. The following is a brief description of both CMP operations.
The chemical mechanical planarization operation of dielectric layers are achieved by dissolving the dielectric layer (i.e., the oxide layer) in hot water under pressure so as to create loose polyhydrosilicates. Thereafter, the polyhydrosilicates can be removed from the wafer surface easily. In contrast, the chemical mechanical planarization operation of metallization layers poses a significant challenge, as the ductile characteristic of metals renders excess metallization layer removal almost impossible. In contrast to non-metal elements in which electrons are localized between the atoms, the valence electrons of metal elements are not localized between a pair of atoms and create a xe2x80x9cconductivity zone,xe2x80x9d xe2x80x9celectron cloud,xe2x80x9d or xe2x80x9celectron layer.xe2x80x9d As a result, the free metal ions are attracted to the electron layer. Thus, the resulting metal atoms can be easily moved along the surface of the metallization layer without causing the bond between them and the electron cloud to break. Frequently, this is related to the xe2x80x9cductilexe2x80x9d nature of metals, which herein is referred to as the ability of the attached resulting metal atoms to easily move from their respective equilibrium positions on the surface of the metallization layer without breaking the metallic bond between them and the surface of the metallization layer. As a comparison, in nonmetal elements having localized electrons, normally, the molecular bonding can be easily broken by simply changing the angles of the atoms by 20% to 30%.
As such, to perform CMP operation on the metallization layers, the metallization layers must be converted into chemical compounds having molecular bonding (e.g., oxides, etc.). In another word, the metallic bonding between the resulting metal atoms and the metallization layer must be changed into a form of molecular bonding wherein electrons are localized between two specific atoms. Thus, in metal CMP operations, metallization layers are oxidized, thereby creating oxidized layers. As the oxide molecules have molecular bonding, the oxidized layers can be easily removed mechanically.
An exemplary prior art CMP system 100 is shown in FIG. 1. The CMP system 100 of FIG. 1 is a belt-type system, so designated because the preparation surface is an endless polishing pad 108 mounted on two drums 114 which drive the polishing pad 108 in a rotational motion as indicated by polishing pad rotation directional arrows 116. A wafer 102 is mounted on a carrier 104. The carrier 104 is rotated in direction 106. The rotating wafer 102 is then applied against the rotating polishing pad 108 with a force F to accomplish a CMP process. Some CMP processes require significant force F to be applied. A platen 112 is provided to stabilize the polishing pad 108 and to provide a solid surface onto which to apply the wafer 102. Depending on the type of excess material being removed, a slurry 118 including of an aqueous solution such as NH4OH or DI water containing dispersed abrasive particles is introduced upstream of the wafer 102.
As in metal CMP operations the metallic layer must first be oxidized, the composition of slurry 118 is an important aspect of the metal CMP operations. In addition, the utilized slurry 118 must be chosen such that the slurry 118 would not induce corrosion and imperfections onto the wafer surface 102. As such, typical metal CMP slurries contain oxidizers and acids, each of which facilitates the conversion of metallization layers into oxide layers. Conventionally, these slurries are designed to be xe2x80x9chighly stablexe2x80x9d and as such have two basic characteristics. First, they have a sufficiently long shelf lifetime. Second, they require a significantly high amount of energy to be activated. As such, the highly stable characteristic of metal slurries yields a significantly low oxidation rate, hence reducing the overall removal rate. As a result, the overall time expended in the metal CMP process is significantly increased thereby reducing the throughput.
In view of the foregoing, a need therefore exists in the art for an enhanced chemical mechanical planarization system that yields a higher throughput utilizing conventional slurries.
Broadly speaking, the present invention fills these needs by manipulating the composition of slurry to enhance the removal rate of excess layers formed on a wafer surface. In one embodiment, the throughput of a chemical mechanical planarization (CMP) system is increased by enhancing the removal rate of excess layer via activation of an implemented slurry. In preferred embodiments, the removal rate of a wafer surface metallization layer is increased in a self-inhibiting CMP system through light induced slurry activation. Self-inhibiting CMP system is herein defined as a CMP system wherein the rate of oxide formation is greater than the rate of metal-oxide dissolution. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a chemical mechanical planarization (CMP) apparatus, is disclosed. The CMP apparatus includes a polishing pad configured to receive a slurry chemical. Also included in the CMP apparatus is a carrier head configured to hold a wafer having a metal surface layer. The carrier head and the polishing pad are configured to mechanically interface during polishing of the metal surface layer with the slurry chemical. The CMP apparatus further includes a radiation unit that is to be applied over the polishing pad at a location after a slurry delivery location and before the pad enters underneath the wafer. The radiation unit is designed to expose the slurry chemical to radiation just prior to the mechanical interface between the metal surface layer of the wafer and the polishing pad.
In another embodiment, a chemical mechanical planarization (CMP) apparatus is disclosed. The CMP apparatus includes a polishing pad designed to rotate and a carrier configured to hold a wafer to be polished. The CMP apparatus further includes a conditioning pad arranged beside the carrier. The polishing pad is designed to move while rotating between being partially over the carrier and the conditioning pad to progressively being completely off of the carrier and completely over the conditioning pad. Also included in the CMP apparatus is a slurry delivery unit. The slurry delivery unit is designed to apply slurry over the conditioning pad so that when rotating, the polishing pad can apply the slurry between the wafer and the polishing pad. The CMP apparatus also includes a radiation source designed to apply radiation to the slurry that is to be applied between the wafer and the polishing pad.
In still a further embodiment, a method for enhancing the removal of a wafer layer of a wafer in chemical mechanical planarization (CMP) systems is provided. The method includes applying radiation to an amount of slurry before the slurry is applied to the wafer layer.
In yet another embodiment, a chemical mechanical planarization (CMP) apparatus, is disclosed. The CMP apparatus includes a polishing pad configured to receive a slurry chemical. Also included in the CMP apparatus is a carrier head configured to hold a wafer having a metal surface layer. The carrier head and the polishing pad are configured to mechanically interface during polishing of the metal surface layer with the slurry chemical. The CMP apparatus further includes a radiation unit that is to be applied over the polishing pad at a location after a slurry delivery location. The radiation unit is designed to expose the slurry chemical to radiation just prior to the mechanical interface between the metal surface layer of the wafer and the polishing pad.
The advantages of the present invention are numerous. Most notably, instead of implementing slurry as is, the present invention activates the slurry in situ via radiation, thus enhancing the removal rate of metal in a CMP system. Thus, the present invention facilitates and expedites the crossing of the activation barrier of the chemical reaction between the slurry and excess layer by introducing energy through applied radiation, e.g., UV or IR exposure. As such, by implementing the same quantity of conventional slurry and within the same period of time, the embodiments of the present invention can be made to yield higher throughputs without contaminating the wafer surface. In one embodiment, a multi-segment light source configuration is beneficial as it allows the CMP system to employ different removal rates on different portions of the wafer being processed, thus achieving a desired planarization profile for the wafer surface. The multi-segment light source configuration also has the capability of redistributing the activation efficiency of present invention and can selectively increase the rate of removal of one portion of semiconductor being polished in relation to others. Another advantage of the embodiments of the present invention is that more stable oxidizers and even gaseous oxygen can be implemented, thus allowing the design of slurries having improved shelf lifetimes.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.